While both are solid, the x60 line is better under transient loads. The one issue with the x50 line is that it doesn't do so well in that department. It's not huge in most cases, but it is the one weakness in an otherwise perfect power supply.
The x60 series addresses that issue. If you can get an x50 series on a great deal, it's still worth doing (I did it). However, if the X50 and x60 are similarly priced, get the x60.
Core i7-2600K @4.2GHz, GIGABYTE GA-Z68XP-UD5, 16GB (4x4) G.Skill RipJaws PC1866
Corsair 650D case, Seasonic X750 Gold PSU
WD `Raptor 600GB boot, WD Caviar Black 1TB data, NEC 7200 DVDRW
2x XFX R9 280X Black in Crossfire, X-Fi Titanium, Dell 2407WFP